This invention relates to a novel offshore platform apparatus and a method for transporting and stationing the same upon the bed of a body of water.
In the past, offshore platforms or towers have been extensively utilized around and upon the continental shelf regions of the world. Examples of offshore platform installations include supports for radar stations, light beacons, scientific and exploration laboratories, chemical plants, power generating plants, etc. Principally, however, offshore platforms have been used by the oil and gas industry in connection with oil and gas drilling, production and distribution operations.
While initial oil and gas operations were conducted along the near shore portions of the Gulf of Mexico, in relatively shallow water depths ranging from swamp or marsh land to 100 or more feet of water, more recent activity has extended to greater water depths of from a few hundred to a thousand or more feet. As deeper water fields are explored and developed, platforms have become larger and environmental loading has become exacerbated. Moreover, production platforms, and in some instances drilling units as well, remain on station for indefinite periods of time and thus encounter prolonged, high stress, periodic wave loading. Accordingly, not only must platforms be capable of withstanding ocean storm conditions; but, minimizing fatigue failure constitutes a significant design consideration.
The foundation of conventional, fixed, offshore structures may be broadly classified in two categories: (1) pile support structures and (2) gravity base structures.
A pile supported structure is one that is attached to the seabed by means of piling driven into the sea floor to support the tower and resist environmental side loading which tends to overturn the structure. Gravity base structures are designed to remain on location strictly because the weight of the structure imposes sufficient loading on the seabed to render the structure safe from sliding or overturning. Gravity base structures do not require pilings and the formation is normally referred to as a mat.
The subject invention is directed to a gravity base platform and a method for facilely constructing, transporting, and stationing the platform upon the bed of a body of water.
One previously known gravity base tower design comprises a concrete platform which was engineered to be installed in the North Sea. In this regard, generally massive concrete structures were utilized to prevent overturning moments from creating an uplift situation on one edge of the base. While concrete designs may solve overturning difficulties, such units are typically bulky, extremely heavy, and difficult to bring on station and reposition if desired.
The mobility of gravity base towers was significantly enhanced by the development of a platform having a generally open tubular superstructure including a base region with caissons secured at peripheral points about the base of the tower. These units were designed to be floated out to a site on the caissons. The caissons would then be controllably flooded to lower the tower to a drilling or production station. Once drilling or production was completed, ballast would be ejected from the caissons and the tower would be buoyantly raised for towing to another site.
Although peripherally stationed caissons may be sufficient to raise and lower a platform, the afloat stability determines the caisson diameter and the stability requirements during a lowering operation determines the height of the caissons. Additionally, if the platform deck and equipment are mounted on the tower before towing to sea, the center of gravity of the overall structure is raised which compounds the stability problem. On the other hand if the deck and associated equipment are installed at sea, after the platform is set, expensive derrick barges and offshore construction equipment are needed to complete construction. As previously noted the foregoing stability situation dictates caisson design and preempts attention to optimizing soil loading. Still further, such previously known units require a high degree of tubular superstructure to support the caissons. Utilization of a high percentage of tubular structures tends to make construction difficult, specialized and not easily performed at conventional shipyard facilities. Yet further, although open superstructure designs are relatively lightweight, such designs tend to be more flexible than concrete designs and exhibit a higher natural period.
Another previously known gravity base design entails a steel base or hull operable to receive ballast on station. Such units are normally lighter than corresponding concrete designs and easier to tow to a site than a generally open superstructure and caisson type base. As will be discussed more fully below, however, steel mat designs typically require a mat having a large diameter of several hundred feet in order to prevent lateral forces from creating an uplift situation from occuring. Additionally, although the ocean floor is thought of as being generally flat, with such large mats, discontinuities in soil formation may create uneven soil bearing zones.
The difficulties suggested in the preceding are not intended to be exhaustive, but rather are among many which may tend to reduce the effectiveness and owner satisfaction with prior gravity base offshore platform systems. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that gravity base offshore platform systems appearing in the past will admit to worthwhile improvement.